Elastography for cement integrity inspection

ABSTRACT

A method for examining integrity of cement in a wellbore includes deploying an ultrasound transducer within a wellbore. One or more reference ultrasound images of the cement within the wellbore are acquired. A pushing pulse is emitted from the ultrasound transducer to elicit a displacement of the cement within the wellbore. A sequence of ultrasound images is acquired, over time, depicting the displacement of the cement within the wellbore elicited by the pushing pulse. A strain tensor map is generated from a difference between the one or more reference ultrasound images and the acquired sequence of ultrasound images. A degree of integrity of the cement within the wellbore is determined based on the generated strain tensor map.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based on provisional application Ser. No. 61/660,147, filed Jun. 15, 2012, the entire contents of which are herein incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to elastography and, more specifically, to elastography for cement integrity inspection.

DISCUSSION OF THE RELATED ART

Cement is widely used in deep water oil drilling as a means of containing pressure in a wellbore after it is drilled and prior to harvesting hydrocarbons. Cement is also used to fill the annular space between the borehole and the casing placed therein. Cement is also used, at times, as a well plug. Under the conditions associated with such drilling, proper wellbore cement integrity may be vital to prevent disaster. If channels form within the wellbore cement, communication between different formations may be possible and these communications may lead to a potentially catastrophic blow out.

Additionally, the interfaces between cement and the borehole and casing are part of the barrier system and may be a pathway for potential leakage.

For these and other reasons, there is a need for accurate detection of cracks, flaws, fissures, mudcake, etc. within wellbore cement. However, existing techniques have shown to be less than completely effective.

SUMMARY

A method for examining integrity of cement in a wellbore includes deploying an ultrasound transducer within a wellbore. A pushing pulse is emitted from the ultrasound transducer to elicit a displacement of the cement within the wellbore. A sequence of ultrasound images is acquired, over time, depicting the displacement of the cement within the wellbore elicited by the pushing pulse. A strain tensor map is generated from the acquired sequence of ultrasound images. A degree of integrity of the cement is determined within the wellbore based on the generated strain tensor map.

The pushing pulse emitted from the ultrasound transducer may elicit a displacement of the cement within a range of about ten to about one hundred microns, depending on the ultrasonic frequency used and the pushing pulse energy.

The ultrasound transducer used to emit the pushing pulse may also be used to acquire the sequence of ultrasound images to estimate the displacement response to the pushing pulse.

Determining the degree of integrity of the cement within the wellbore based on the generated strain tensor map may include identifying one or more subregions within the cement based on disparities within the stain tensor map, characterizing each subregion within the cement based on values of the strain tensor map for each subregion and known strain tensors for various materials including cement, fluid and gas, and determining if there is one or more channels present within the cement based on the characterizations of the subregions.

When the integrity of the cement within the wellbore is determined to be sufficiently poor, remedial actions are performed to improve the integrity of the cement. The remedial action may include agitating the cement within the wellbore before the cement hardens. Agitation may include application of a focused acoustic beam. The focused acoustic beam may cause liquefaction within the cement.

A method for deploying cement in a wellbore includes pumping cement into a wellbore. An ultrasound transducer is deployed into the wellbore. A pushing pulse is emitted from the ultrasound transducer to elicit a displacement of the cement within the wellbore. A sequence of ultrasound images is acquired, over time, depicting the displacement of the cement within the wellbore elicited by the pushing pulse. A strain tensor map is generated from the acquired sequence of ultrasound images. The presence of channels within the cement within the wellbore is detected based on the generated strain tensor map. The cement within the wellbore is agitated to reduce the channels when it is determined that channels are present within the cement.

Additional cement may be pumped into the wellbore after the cement already pumped into the wellbore has been sufficiently agitated to reduce the channels.

The pushing pulse emitted from the ultrasound transducer may elicit a displacement of the cement within a range of about ten to about one hundred microns depending on the ultrasonic frequency used and the pushing pulse energy.

The ultrasound transducer used to emit the pushing pulse may also be used to acquire the sequence of ultrasound images to estimate the displacement response to the pushing pulse.

Detecting the presence of channels within the cement within the wellbore based on the generated strain tensor map may include identifying one or more subregions within the cement based on disparities within the stain tensor map, characterizing each subregion within the cement based on values of the strain tensor map for each subregion and known strain tensors for various materials including cement, fluid and gas, and determining if there is one or more channels present within the cement based on the characterizations of the subregions.

A computer system includes a processor and a non-transitory, tangible, program storage medium, readable by the computer system, embodying a program of instructions executable by the processor to perform method steps for examining integrity of cement in a wellbore. The method includes controlling a deployment of an ultrasound transducer within a wellbore. An emission of a pushing pulse from the ultrasound transducer is controlled to elicit a displacement of the cement within the wellbore. A sequence of ultrasound images is acquired, over time, depicting the displacement of the cement within the wellbore elicited by the pushing pulse. A strain tensor map is generated from the acquired sequence of ultrasound images. A degree of integrity of the cement within the wellbore is determined based on the generated strain tensor map.

The pushing pulse emitted from the ultrasound transducer may elicit a displacement of the cement within a range of about ten to about one hundred microns depending on the ultrasonic frequency used and the pushing pulse energy.

The ultrasound transducer used to emit the pushing pulse may also be used to acquire the sequence of ultrasound images to estimate the displacement response to the pushing pulse.

Determining the degree of integrity of the cement within the wellbore based on the generated strain tensor map may include identifying one or more subregions within the cement based on disparities within the stain tensor map, characterizing each subregion within the cement based on values of the strain tensor map for each subregion and known strain tensors for various materials including cement, fluid and gas, and determining if there is one or more channels present within the cement based on the characterizations of the subregions.

When the integrity of the cement within the wellbore is determined to be sufficiently poor, remedial actions may be performed to improve the integrity of the cement. The remedial action may include agitating the cement within the wellbore before the cement hardens.

A computer system includes a processor and a non-transitory, tangible, program storage medium, readable by the computer system, embodying a program of instructions executable by the processor to perform method steps for deploying cement in a wellbore. The method includes controlling a pumping of cement into a wellbore, controlling a deployment of an ultrasound transducer into the wellbore. An emission of a pushing pulse or train of pulses from the ultrasound transducer is controlled to elicit a displacement of the cement within the wellbore. A sequence of ultrasound images is acquired, over time, depicting the displacement of the cement within the wellbore elicited by the pushing pulse. A strain tensor map is generated from the acquired sequence of ultrasound images. The presence of channels within the cement within the wellbore is detected based on the generated strain tensor map. An agitating of the cement within the wellbore is controlled to reduce the channels when it is determined that channels are present within the cement.

Additional cement may be pumped into the wellbore after the cement already pumped into the wellbore has been sufficiently agitated to reduce the channels.

The pushing pulse emitted from the ultrasound transducer may elicit a displacement of the cement within a range of about ten to about one hundred microns.

The ultrasound transducer used to emit the pushing pulse may also be used to acquire the sequence of ultrasound images.

Detecting the presence of channels within the cement within the wellbore based on the generated strain tensor map may include identifying one or more subregions within the cement based on disparities within the stain tensor map, characterizing each subregion within the cement based on values of the strain tensor map for each subregion and known strain tensors for various materials including cement, fluid and gas, and determining if there is one or more channels present within the cement based on the characterizations of the subregions.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present disclosure and many of the attendant aspects thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram illustrating an apparatus for performing cement integrity inspection using elastography in accordance with exemplary embodiments of the present invention;

FIG. 2 is a flow chart illustrating a method for performing cement integrity inspection using elastography in accordance with exemplary embodiments of the present invention;

FIG. 3 is a flow chart illustrating an approach for performing cement deployment in accordance with exemplary embodiments of the present invention; and

FIG. 4 shows an example of a computer system capable of implementing the method and apparatus according to embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE DRAWINGS

In describing exemplary embodiments of the present disclosure illustrated in the drawings, specific terminology is employed for sake of clarity. However, the present disclosure is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents which operate in a similar manner.

Exemplary embodiments of the present invention seek to provide approaches for detecting imperfections such as cracks, flaws, fissures, mudcake, etc. within wellborn cement using elastography. Elastrography is a method for imaging soft tissue within the human body to differentiate between tumors and healthy tissue. Here, mechanical compression or vibration is applied to the tissue under study and tissue stiffness elasticity is gauged by ultrasonic imaging. As tumorous tissue is understood to have a greater stiffness than healthy tissue, tumors may be detected and evaluated. The degree of elasticity/stiffness may be measured, for example, by measuring tissue displacement and building a strain tensor map which describes the displacement across the tissue. The strain tensor map may thereafter be used to distinguish different subregions of tissue based on their elasticity.

Exemplary embodiments of the present invention adapt elastography techniques to the imaging of wellbore cement. However, rather than detecting tumors within healthy tissue, exemplary embodiments of the present invention operate on the understanding that channels within the cement may contain mud, liquids, or gasses, which may have different elasticity than the cement itself.

Exemplary embodiments of the present invention may deploy an ultrasound device down the casing, for example, via a wireline. The ultrasound device may include an ultrasound transducer that may emit a pushing pulse. The pushing pulse may then provoke a displacement throughout the cement on the order of 10-100 microns. The ultrasound device may immediately thereafter image the cement material beyond the casing and capture displacement over time throughout the cement. A strain map may then be computed from the acquired displacement data. As the elasticity properties of mudcake, drilling mud, cured cement, air, and water may be sufficiently distinct, one or more different regions may be observable from the strain map and accordingly, one or more channels may be automatically detected therefrom.

The cement so examined may either be cement located outside of the well casing and inside the wellbore walls, for example, primary cementing, or the cement so examined may fill the interior of the casing, for example, a well plug. Moreover, the techniques discussed herein may be applied to imaging all examples of cement inside and beyond the well casing.

FIG. 1 is a schematic diagram illustrating an apparatus for performing cement integrity inspection using elastography in accordance with exemplary embodiments of the present invention. FIG. 2 is a flow chart illustrating a method for performing cement integrity inspection using elastography in accordance with exemplary embodiments of the present invention.

During a drilling phase, a drilling pipe is lowered from a platform or rig 11. The drilling pipe may be lowered from the water surface 12 down past the seafloor 13. A drill bit (not shown) may perform a drilling operation to displace the seabed and create the wellbore 14. As seabed is displaced from the wellbore 14, the drilling pipe may be lowered an additional distance. As a deep water drilling operation may extend for many thousands of feet below sea level, the wellbore 14 may come under considerable pressure. Moreover, the wellbore 14 may communicate with hydrocarbon deposits, such as accumulations of petroleum and/or methane, which may be collected. However, collection of the hydrocarbons may involve removing the drilling apparatus from the wellbore 14 and setting up collecting apparatus. As this transition may take a long time, to counter the pressure encountered and to prevent the hydrocarbons from flowing up the wellbore 14 before the collecting apparatus may be deployed, a drilling mud may be deployed into the wellbore 14 as the drilling is being performed and the drilling mud may be replaced with cement 15 while the drilling apparatus is being removed from the wellbore 14 and prior to the deployment of the collecting apparatus.

Cement also has a variety of other applications within deep water drilling. For example, cement may be placed within the annular void between the well casing and the wellbore. Cement may also be used for zonal isolation (e.g. hydraulic bond) to prevent migration of fluids up the wellbore, to support the axial load of the casing (e.g. shear bond), and to support the borehole and prevent formation collapse.

The cement 15, which may be delivered through a riser pipe that is lowered into the wellbore 14, may play an important role in stabilizing the wellbore 14 before the collecting apparatus can be successfully deployed. If the integrity of the cement 15 is not assured, the hydrocarbons may travel up the wellbore 14 causing a potentially catastrophic blowout at the rig or platform 11.

Accordingly, exemplary embodiments of the present invention deploy an ultrasound transducer 17 down into the wellbore 14 via a wireline 16 (Step S21). The ultrasound transducer 17, or another imaging device, may be used to acquire one or more reference ultrasound images of the cement 15 within the wellbore 14 (Step S22) imaging the cement 15 prior to the application of the pushing pulse. The ultrasound transducer 17 may then emit a pushing pulse (Step S23). The pushing pulse may be sufficient to create a displacement within the cement 15.

For example, a displacement on the order of between 10 and 100 microns within the cement 15 may be created by the pushing pulse. The transducer 17, or the other imaging device, may then perform an ultrasound imaging of the cement 15 (Step S24). The ultrasound imaging may include the acquisition of a series of images over time. A strain tensor map describing the deformation of the imaged cement 15 may then be calculated from the ultrasound imaging (Step S25), for example, by determining a difference between the reference images taken before the pushing pulse and the ultrasound images taken after the pushing pulse. The resulting strain tensor map may then be analyzed to identify a plurality of subregions within the images cement 15 (Step S26). Each of the identified subregions may then be characterized based on a degree of displacement caused by the pushing pulse that is observed from the ultrasound imaging (Step S27). Characterization of the subregions may include determining whether the subregion is solid cement or channels containing drilling mud, liquid or gas, as each substance may have a known degree of displacement, given the pushing pulse. From the characterization of the subregions, it may be determined whether the cement has been effectively deployed (Step S28).

The above-described procedure may be performed during the deployment of the cement to ensure an effective deployment that is free of channels. FIG. 3 is a flow chart illustrating an approach for performing cement deployment in accordance with exemplary embodiments of the present invention.

Cement may be deployed to a lower section wellbore (Step S31). Prior to the hardening of the deployed cement, an elastographic study may be performed on the cement so-deployed (Step S32). The elastographic study may be performed substantially as described above with reference to FIGS. 1 and 2. It may then be determined, based on the elastographic study, whether the deployed and unhardened cement is substantially free of channels (Step S33). If the deployed cement is determined to be substantially free of channels based on the elastographic study (Yes, Step S33), then deployment of cement may continue (Step S31). The continued deployment of the cement may include the filling of a next section of the wellbore. However, deployment of the cement may be continuous from step to step.

If, however, the results of the elastographic study show that the deployed cement is not substantially free of channels (No, Step S33), then remedial action may be conducted (Step S34). Remedial action may include, for example, agitating the deployed wet cement. While the pouring of the cement may be continuous for as long as the cement is substantially free of channels, once channels are detected, the pouring of the cement may be stopped or slowed down while remedial action is performed until the cement is deemed to be substantially free of channels. Thus, after the agitation of the cement (Step S34), or perhaps while it is being conducted, the elastographic study may be repeated (Step S32) until the cement is substantially free of channels (Yes, Step S33), at which point deployment may continue (Step S31) until the cement is fully deployed.

Agitation of the cement and other remedial action may include the use of one or more acoustic beams. The acoustic beams may be, for example, high intensity focused ultrasound (HIFU). Acoustic beams may be highly localized and may have the ability to impart localized vibrations to cause liquefaction of the cement in and around the vicinity of the channels in a phenomenon that is similar to soil liquefaction, in which seismic activity causes hard ground to obtain a nearly liquid consistency. Liquefaction may be performed to remediate detected channels even where the cement has partially or entirely cured. The acoustic beams may be produced by one or more transducers, for example, by an array of transducers. A highly focused region of maximum acoustic impact may be directed and adjusted electronically for the desired purpose.

One or more elastographic study pushing pulses may additionally or alternatively be used to effect the ultrasonic remediation. For example, a prior elastographic or simple imaging study may reveal a region needing agitation of cement. A reference ultrasound image may be acquired. A high power focused ultrasonic beam may then be applied to liquefy the cement and cause displacement. An ultrasound image may then be acquired allowing immediate confirmation of whether liquefaction has occurred. High power focused sonication may then resume, and sonication may be interrupted, for example, briefly, to acquire addition monitoring of the progress of remediation. The steps of imaging and high power focused sonication can be repeated continuously providing continuous feedback during the remediation process.

In performing remedial action, the acoustic beam may be pulsed on and off electronically at any desired rate to impart a localized vibration sufficient to cause liquefaction. An optimum frequency for causing liquefaction may be chosen based on prior knowledge of the cement mechanics or elastographic imaging, such as is described above, may be used to determine when liquefaction has occurred. Once the cement material has been liquefied, it may readily displace in response to the pushing pulse.

As liquefaction may cause cement material above the treated area to give way, remediation at a lower position may give rise to additional channels at a higher point, accordingly, liquefaction may be performed at increasingly upwards positions within the wellbore until all cement therein is free of channels.

Where the deployment is deemed to be substantially free of channels (Yes, Step S33) and deployment of cement continues to a next section (Step S31), performance of the elastographic study (Step S32) may include raising the ultrasound transducer to the next section so that the newly deployed cement may be tested, for example, in the manner discussed above.

Where the cement is determined to not be substantially free of channels (No, Step S34) and the remedial action conducted is not successful in substantially eliminating the channels, additional remedial action may be conducted including, for example, closing off the wellbore and ending the drilling so that a potentially catastrophic blowout does not occur.

Control of the above-described method for performing cement deployment in accordance with exemplary embodiments of the present invention may be managed by a computer system running specially designed software. FIG. 4 shows an example of a computer system which may implement a method and system of the present disclosure. The system and method of the present disclosure may be implemented in the form of a software application running on a computer system, for example, a mainframe, personal computer (PC), handheld computer, server, etc. The software application may be stored on a recording media locally accessible by the computer system and accessible via a hard wired or wireless connection to a network, for example, a local area network, or the Internet.

The computer system referred to generally as system 1000 may include, for example, a central processing unit (CPU) 1001, random access memory (RAM) 1004, a printer interface 1010, a display unit 1011, a local area network (LAN) data transmission controller 1005, a LAN interface 1006, a network controller 1003, an internal bus 1002, and one or more input devices 1009, for example, a keyboard, mouse etc. As shown, the system 1000 may be connected to a data storage device, for example, a hard disk, 1008 via a link 1007.

Exemplary embodiments described herein are illustrative, and many variations can be introduced without departing from the spirit of the disclosure or from the scope of the appended claims. For example, elements and/or features of different exemplary embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. 

What is claimed is:
 1. A method for examining integrity of cement in a wellbore, comprising: deploying an ultrasound transducer within a wellbore; acquiring one or more reference ultrasound images of the cement within the wellbore; emitting a pushing pulse from the ultrasound transducer to elicit a displacement of the cement within the wellbore; acquiring a sequence of ultrasound images, over time, depicting the displacement of the cement within the wellbore elicited by the pushing pulse; generating a strain tensor map from a difference between the one or more reference ultrasound images and the acquired sequence of ultrasound images; and determining a degree of integrity of the cement within the wellbore based on the generated strain tensor map.
 2. The method of claim 1, wherein the pushing pulse emitted from the ultrasound transducer elicits a displacement of the cement within a range of about ten to about one hundred microns.
 3. The method of claim 1, wherein the ultrasound transducer used to emit the pushing pulse is also used to acquire the sequence of ultrasound images.
 4. The method of claim 1, wherein determining the degree of integrity of the cement within the wellbore based on the generated strain tensor map includes: identifying one or more subregions within the cement based on disparities within the stain tensor map; characterizing each subregion within the cement based on values of the strain tensor map for each subregion and known strain tensors for various materials including cement, fluid and gas; and determining if there is one or more channels present within the cement based on the characterizations of the subregions.
 5. The method of claim 1, wherein when the integrity of the cement within the wellbore is determined to be sufficiently poor, remedial actions are performed to improve the integrity of the cement.
 6. The method of claim 5, wherein the remedial action includes agitating the cement within the wellbore before the cement hardens or pumping additional cement into the wellbore.
 7. The method of claim 6, wherein agitation includes application of a focused acoustic beam.
 8. The method of claim 7, wherein the focused acoustic beam causes liquefaction within the cement.
 9. A method for deploying cement in a wellbore, comprising: pumping cement into a wellbore; deploying an ultrasound transducer into the wellbore; emitting a pushing pulse from the ultrasound transducer to elicit a displacement of the cement within the wellbore; acquiring a sequence of ultrasound images, over time, depicting the displacement of the cement within the wellbore elicited by the pushing pulse; generating a strain tensor map from the acquired sequence of ultrasound images; detecting the presence of channels within the cement within the wellbore based on the generated strain tensor map; and agitating the cement within the wellbore to reduce the channels when it is determined that channels are present within the cement.
 10. The method of claim 9, wherein agitation includes application of a focused acoustic beam.
 11. The method of claim 10, wherein the focused acoustic beam causes liquefaction within the cement.
 12. The method of claim 9, wherein the pushing pulse emitted from the ultrasound transducer elicits a displacement of the cement within a range of about ten to about one hundred microns.
 13. The method of claim 9, wherein detecting the presence of channels within the cement within the wellbore based on the generated strain tensor map includes: identifying one or more subregions within the cement based on disparities within the stain tensor map; characterizing each subregion within the cement based on values of the strain tensor map for each subregion and known strain tensors for various materials including cement, fluid and gas; and determining if there is one or more channels present within the cement based on the characterizations of the subregions.
 14. A computer system comprising: a processor; and a non-transitory, tangible, program storage medium, readable by the computer system, embodying a program of instructions executable by the processor to perform method steps for examining integrity of cement in a wellbore, the method comprising: controlling a deployment of an ultrasound transducer within a wellbore; controlling an emission of a pushing pulse from the ultrasound transducer to elicit a displacement of the cement within the wellbore; acquiring a sequence of ultrasound images, over time, depicting the displacement of the cement within the wellbore elicited by the pushing pulse; generating a strain tensor map from the acquired sequence of ultrasound images; and determining a degree of integrity of the cement within the wellbore based on the generated strain tensor map.
 15. The computer system of claim 14, wherein the pushing pulse emitted from the ultrasound transducer elicits a displacement of the cement within a range of about ten to about one hundred microns.
 16. The computer system of claim 14, wherein determining the degree of integrity of the cement within the wellbore based on the generated strain tensor map includes: identifying one or more subregions within the cement based on disparities within the stain tensor map; characterizing each subregion within the cement based on values of the strain tensor map for each subregion and known strain tensors for various materials including cement, fluid and gas; and determining if there is one or more channels present within the cement based on the characterizations of the subregions.
 17. The computer system of claim 14, wherein when the integrity of the cement within the wellbore is determined to be sufficiently poor, remedial actions are performed to improve the integrity of the cement.
 18. The computer system of claim 17, wherein the remedial action includes agitating the cement within the wellbore before the cement hardens.
 19. The computer system of claim 18, wherein agitation includes application of a focused acoustic beam.
 20. The computer system of claim 19, wherein the focused acoustic beam causes liquefaction within the cement.
 21. A computer system comprising: a processor; and a non-transitory, tangible, program storage medium, readable by the computer system, embodying a program of instructions executable by the processor to perform method steps for deploying cement in a wellbore, the method comprising: controlling a pumping of cement into a wellbore; controlling a deployment of an ultrasound transducer into the wellbore; controlling an emission of a pushing pulse from the ultrasound transducer to elicit a displacement of the cement within the wellbore; acquiring a sequence of ultrasound images, over time, depicting the displacement of the cement within the wellbore elicited by the pushing pulse; generating a strain tensor map from the acquired sequence of ultrasound images; detecting the presence of channels within the cement within the wellbore based on the generated strain tensor map; and controlling an agitating of the cement within the wellbore to reduce the channels when it is determined that channels are present within the cement.
 22. The computer system of claim 20, wherein the pushing pulse emitted from the ultrasound transducer elicits a displacement of the cement within a range of about ten to about one hundred microns.
 23. The computer system of claim 20, wherein detecting the presence of channels within the cement within the wellbore based on the generated strain tensor map includes: identifying one or more subregions within the cement based on disparities within the stain tensor map; characterizing each subregion within the cement based on values of the strain tensor map for each subregion and known strain tensors for various materials including cement, fluid and gas; and determining if there is one or more channels present within the cement based on the characterizations of the subregions.
 24. The computer system of claim 21, wherein agitation includes application of a focused acoustic beam.
 25. The computer system of claim 24, wherein the focused acoustic beam causes liquefaction within the cement. 